Half-duplex (HDX) radio frequency communication is a way of communication for a passive RFID transponder. In the communication process, information can be transmitted by an RFID reader to the transponder, referred to as downlink transmission, or by the transponder to the reader, referred to as uplink transmission. The HDX transmission is such a transmission mode in which uplink transmission and downlink transmission are not simultaneously performed. During a downlink transmission, the reader transmits radio frequency (RF) field energy, and a passive transponder receives the RF field energy with an L-C resonance circuit formed by a resonance inductor L (also called antenna) and a resonance capacitor C; a rectifier circuit in the transponder converts an alternating current into a direct current to be used by internal circuitries of the transponder, and stores electrical energy obtained through rectification in a storage capacitor CL. Mostly, amplitude modulation (AM) is adopted as a modulation scheme for the information delivered by the reader to the transponder, that is, RF oscillation current signals with different amplitudes are used to represent the code “0” or “1” in digital transmission. The RF signal in the downlink transmission may include operational instructions, such as writing an identification (ID) code into the memory of the transponder. In such operation, the transponder first receives the RF signal, and then performs a demodulation operation, and finally carries out the operation of writing the ID code into the memory. The transponder responds to the reader with an uplink transmission. During the uplink transmission, the reader stops transmitting RF field energy, a state referred to as field-off; and the transponder is powered by the electrical energy stored in an energy-storage capacitor, that is, the transponder transmits its response to the reader through the antenna. In some international standard RFID communications protocols, such as the ISO 11784/11785 standard for animal identification, transponder's response may be modulated using a frequency shift keying (FSK) scheme, where code “0” and code “1” in information may be respectively represented by different signal frequencies.
A key performance requirement of RFID applications is the distance of communication, i.e., communication with high sensitivity. As far as the HDX communication is concerned, the key is to maintain oscillation of the L-C resonance circuit at the antenna end during the period when RF field is switched off. Provided that oscillation magnitude is sufficient as required by a reader's receiving sensitivity, the oscillation frequency of the L-C resonance circuit is determined by the data “0” and “1” being sent; the oscillation has to be maintained until the uplink of data is completed. In this communication mode, energy stored in the storage capacitor is mainly consumed by the oscillation maintenance circuit, while other circuit modules are in a sleep state, consuming very little electrical energy. Apparently, the design quality of oscillation maintenance circuit is one of the key techniques for passive HDX RFID transponders. An oscillation maintenance circuit that charges an L-C resonance circuit through an energy-storage capacitor instead of an external power supply significantly influences communication distance of the transponder.
FIG. 1 is a block diagram of a communication process of an HDX passive RFID transponder. The oscillation maintenance circuit shown in the figure is one of the key elements. The application is an improvement on this circuit.
An existing solution for the oscillation maintenance circuit is disclosed in U.S. Pat. No. 6,806,738 by Texas Instruments, which proposed a complex peak detection circuit to monitor peak value of the alternating current oscillation magnitude when RF field is off. When the detected peak of alternating current oscillation magnitude is lower than a pre-determined threshold, an oscillation maintenance circuit controls related circuitry to inject electrical charges stored in a storage capacitor into the L-C resonance circuit, so that the resonance circuit continues to oscillate while maintaining a certain level of oscillating amplitude. As shown in FIG. 2, a current pulse is generated just at the point of time when the peak value of an RF signal is lower than the pre-determined threshold. This method is referred to as “plucking (fast injection)”.
An advantage of the fast injection is that the oscillation maintenance circuit has very high efficiency. When the resonance circuit requires energy, the oscillation maintenance circuit injects a current into the resonance inductor, and maintains oscillation with high-energy current pulses. However, it has the following two disadvantages:
First, the signal processing process is complicated, which results in a complicated circuit structure whose implementation requires a lot of analog circuitries. Those analog circuitries themselves consume a lot of energy in the energy-storage capacitor, which in turn limits the usage of the fast injection method.
Second, the frequency of the injected current pulses is determined by the decaying characteristic of the oscillation magnitude of the resonance circuit, and is significantly different from the resonance frequency of the resonance circuit without correlation. Therefore, during uplink, there is a frequency shift on the oscillating signals at the antenna end, causing difficulty for the reader to receive and demodulate uplinked signals.
Another existing solution of the oscillation maintenance circuit is disclosed in U.S. Pat. No. 7,667,548 by Texas Instruments, which employed an end-of-burst (EOB) detection circuit. When an end of burst of a reader is detected, the EOB detection circuit generates an enable signal that in turn controls the oscillation maintenance circuit to continue to operate. In this solution, the oscillation maintenance circuit comprises a clock generation circuit, a programmable memory, an AND gate circuit, a current-limiting resistor, and a switch, etc. The clock generation circuit extracts a clock signal out of the RF field signal, and a combinational logic operation is performed between the clock signal and the “0” and “1” bit stream of the data being transmitted. In such a way, a current injection switch is turned on and off, and a current is smoothly injected into an L-C resonant circuit. Further, the current injection occurs in a negative half-period of the RF signal, and the injected current is a varying current without a fixed value. The injected current is limited by the current-limiting resistor R, and is related to an intrinsic quality factor of the resonance circuit, and therefore needs to be designed and controlled carefully. Further, a plurality of branches comprising of series connected current-limiting resistor R and current injection switch can be used to calibrate the resistance deviations due to manufacturing process deviations.
Advantages of this method are that the current can be smoothly injected, without causing a frequency drift of the RF response signal generated by the resonance circuit, and that circuit structure is simpler than that of the first solution and is easy to implement. However, there are disadvantages as follows:
First, the efficiency is low. Current injection occurs in the entire negative half-period of the RF signal, and takes a longer time than with current pulses, and therefore current consumption is high.
Second, the amount of the injected current in this method is closely related to quality factor of the resonance circuit. Therefore, the current-limiting resistor R needs to be designed carefully. The circuit uses a plurality of current injection branches controlled by switches, and it is a complicated process to select and control a current injection branch.
A third existing solution is disclosed in U.S. Pat. No. 8,629,759 by Texas Instruments. In this solution, modifications are made based on the previous two solutions. Firstly, regarding the problem of frequency drift in the first solution, a phase-locked loop (PLL) consisted of a loop filter, a phase and frequency detector, a voltage-controlled oscillator, and a multiplexer is adopted to stabilize the signal frequency. Secondly, to address the issue of efficiency in the second solution, a pulse width control method is proposed (that is, current is injected into resonance circuit in every half period, and a duration of current injection may be controlled accordingly). With the combination of the two modifications, a method to generate an injection control pulse signal of a stable frequency and a controllable pulse width to control the “plucking” current can be obtained. This solution works well and resolves the current injection efficiency problem and the frequency drift problem.
However, the third solution does not provide a method to control the pulse width. In addition, a PLL circuit for stabilizing control signal's frequency causes even larger power consumption of the transponder. Thus, the overall performance of the transponder is decreased.